1. Field of the Invention
The present invention relates to a thin film transistor and a method for manufacturing the same, and more particularly to a thin film transistor and a method for manufacturing the same which are capable of reducing a change in a threshold voltage of the thin film transistor by forming a buffer layer with a low dielectric constant.
2. Discussion of Related Art
In general, in order to form a thin film transistor on a flexible substrate, that is, on a metal thin film made of stainless steel, titanium (Ti), etc., an insulation buffer layer must be provided between the metal thin film and the thin film transistor. A thin film transistor having a buffer layer formed on a metal thin film has electrical and structural characteristics similar to a system on insulator (SOI) transistor. In the thin film transistor having the buffer layer form on the metal thin film, a semiconductor layer constituting the thin film transistor is made of low temperature polysilicon (LTPS) obtained by heating and crystallizing an amorphous silicon layer at a low temperature.
In the thin film transistor with the structure described above, that is, in the thin film transistor having the semiconductor layer made of the low temperature polysilicon formed on the metal thin film, a threshold voltage of the thin film transistor can be controlled through a circuit design by effectively utilizing a bias signal (for example, current, voltage, etc.) which is applied to the thin film transistor. In this case, in order to stabilize the operation of the thin film transistor, that is, in order to obtain an optimal result when an arbitrary signal is processed, a direct current (DC) voltage or a predetermined signal can be added to the arbitrary signal to be processed.
However, when a DC voltage or a predetermined signal is applied to the thin film transistor with the structure described above, the threshold voltage of the thin film transistor can be changed. Also, if an unintended voltage is applied to the metal thin film or if unexpected charges are accumulated on the metal thin film due to static electricity, etc., the threshold voltage of the thin film transistor may become different from a reference design value recommended when the thin film transistor is initially designed. Furthermore, if the threshold voltage of the thin film transistor becomes different from the reference design value, color coordinates to be displayed on a screen of a display apparatus utilizing the thin film transistor are changed, and accordingly gray scale and contrast are also changed.
Therefore, in order to solve the above-described problems, the present invention provides a thin film transistor and a method for manufacturing the same which are capable of significantly reducing an electrical characteristic change of the thin film transistor, by in advance preventing an electrical characteristic change (for example, a change in a threshold voltage) of the thin film transistor from occurring when an unintended voltage is temporarily or momentarily applied to the thin film transistor.